1. Field of the Invention
The present invention relates generally to a switch or key on a keypad and, more particularly, to a non-contact switch or key.
2. Description of the Related Art
Keypads or keyboards provide operator interface with household appliances and electronic devices, as well as industrial devices. Keypads may contain one or more switches or keys to allow an operator to enter instructions or information to the household or industrial device. The instructions or information entered may be a simple ON/OFF instruction or a complex combination of alphabetical and numerical information.
A typical switch is a manually or mechanically actuated device for making, breaking or changing the connections in an electrical circuit. A keypad or keyboard has a set of switches or keys arranged systematically and used to operate a piece of equipment. The typical keyboard is a raised keyboard similar to the type used on personal computers. The individual keys or switches are raised above the base of the keyboard. To activate the keys, an operator applies a force to manually depress the key into contact with the base of the keyboard. The main problem with this keyboard is that a force must be directly applied to the individual keys to activate them. Another problem with raised keyboards is that they are difficult to clean and dirt and foreign materials can accumulate in the gaps between the keys. Furthermore, any liquid spilled onto the keyboard seeps into the gaps causing corrosion or keyboard failure. These problems are compounded when this type of keyboard is placed in an environment such as a kitchen where foods and liquids will contact the keyboard.
Flat keyboards have been developed to prevent foreign materials from gathering and seeping between the individual keys. The typical flat keyboard is a membrane keyboard. The membrane keyboard provides a continuous flat layer of plastic material. Individual membrane switches correspond to the individual keys. A membrane switch has a flexible top layer separated from a bottom layer. To activate the switch on the membrane keyboard, a force must be applied to depress the top layer into contact with the bottom layer. The main problem with the membrane keyboard is that a force must be applied to depress the membrane keys. An additional problem with the membrane keyboard is that it is susceptible to damage. The top plastic layer tends to easily scratch or puncture. Additionally, typical cleaning solvents may damage the plastic layer.
Glass keyboards have been developed to provide non-contact key activation. The typical glass keyboard generally senses the proximity of a human finger to a key by measuring the entire human body's capacitance to earth ground. When a human finger comes within a predetermined proximity to a key, the key is activated. These glass keyboards provide the desired non-contact feature, but they have performance drawbacks. The capacitance technique has proven to be unreliable at installations where a good earth ground does not exist. Additionally, electrically noisy environments interfere with the performance of the key. Furthermore, performance is usually compromised by a wide range of behavior from one keyboard to another. Performance is further compromised by large variations in temperature. Not only is the performance of the capacitance glass keyboards lacking, but some systems require expensive and difficult manufacturing methods, such as vapor deposition of conductive regions or placement of components onto glass.
Thus, a need has arisen for new key or keypad which will activate without the application of a force, will prevent foreign materials from gathering between the individual keys, will clean easily, will offer damage protection, and will provide consistent performance.